So again, admit that, to draw a stone, you need a knowledge of geology. I have told you that you do not, but admit it. Do you expect me to teach you, here, the relations between quartz and oxide of iron; or between the Silurian and Permian systems? If you care about them, go to Professor Phillips, and come back to me when you know them. And, in like manner, admit that, to draw a man, you want the knowledge of his bones:--you do not; but admit that you do. Why should you expect me, here, to teach you the most difficult of all the sciences? If you want to know it, go to an hospital, and cut dead bodies to pieces till you are satisfied; then come to me, and I'll make a shift to teach you to draw, even then--though your eyes and memory will be full of horrible things which Heaven never meant you so much as a glance at. But don't expect me to help you in that ghastly work: any more than among the furnaces and retorts in Professor Maskelyne's laboratory. 161. Let us take one more step in the logical sequence. You do not, I have told you, need either chemistry, botany, geology, or anatomy, to enable you to understand art, or produce it. But there is one science which you _must_ be acquainted with. You must very intensely and thoroughly know--how to behave. You cannot so much as feel the difference between two casts of drapery, between two tendencies of line,--how much less between dignity and baseness of gesture,--but by your own dignity of character. But, though this is an essential science, and although I cannot teach you to lay one line beside another rightly, unless you have this science, you don't expect me in these schools to teach you how to behave, if you happen not to know it before!